Coke and Mentos

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Diet Coke and Mentos: What is really behind this physical reaction? Tonya Coffey
*Published in the American Journal of Physics, June 2008

In case you hadn’t heard… Diet Coke and Mentos is a major fad! Subject of countless YouTube videos

1st shown on the Letterman show in 1999 by chemistry teacher Lee Marek

Subject of a 2006 episode of Mythbusters

Why on earth do I care so much about Mentos?
Originally to give my PHY2210 students the experience of participating in a real research project, not a canned lab.

But I admit the project exploded beyond my original vision…

Bubble Theory: Why do sodas fizz when you open them?
We like our drinks nice and fizzy. Thanks, Henry’s Law!

Here P is partial pressure of gas above liquid, K is a constant, and c is the molar concentration of solute. The partial pressure of the gas above the solution is directly proportional to the concentration of solute gas in the solution. So when you pop the top, the equilibrium within the bottle or can is broken, and the concentrated carbon dioxide gas leaves the container. The partial pressure of carbon dioxide in the surrounding gas then drops, forcing the concentration of the solute gas to drop, so it bubbles out.

Bubble theory: How bubbles form in liquids
In most liquids, there is some dissolved gas. In high surface tension liquids, like water, it is tough for bubbles to form, because water molecules like to be next to other water molecules (capillary forces). To overcome this, a nucleation site is generally needed. Gas molecules congregate next to nucleation sites, which break up the network of water molecules. When enough are gathered, they form a bubble. Due to capillary forces, the bubble will initially stay at its nucleation site. But usually, the buoyancy of the bubble will eventually cause it to rise, as more and more gas molecules collect in the bubble. *Liger-Belair and Jeandet, Europhysics News

More fun bubble facts…
When a soda is bottled, it is bottled under a relatively high pressure of CO2 that exceeds the solubility of CO2 in the rest of the formula (mostly water). When the can is opened without shaking high pressure CO2 above the liquid escapes, making the familiar hiss. The CO2 in the liquid slowly escapes until equilibrium is achieved. When the unopened can is shaken, some of the gaseous CO2 gets mixed into the liquid, forming a supersaturated solution. The mixed in gas also provide growth sites for the dissolved CO2. The growth sites allow the CO2 to escape much more rapidly-- hence the "explosive" evolution of CO2 gas.

Bubble Theory: The importance of surfactants
• Adding surfactants to water reduce the work required to form a bubble, making bubbles easier to form and longer lasting • Surfactants are long chained molecules (like soap) that have a water loving and water fearing end

What burning questions did the Mythbusters leave for me?
• Main Mentos contributors to the reaction: gum arabic and gelatin • Main Diet Coke contributors to the reaction: caffeine, apsartame, potassium benzoate • Rough surface of Mentos provides growth sites for the carbon dioxide dissolved in Diet Coke • How rough is Mentos compared to other samples and how much does roughness matter compared to presence of surfactants in soda or candy? • What is relative contribution of ingredients to reaction, and why do they work? Surfactants? • Are there any other factors in play?

It’s not an acid/base reaction!
• Lots of speculation on the web. • We measured pH of soda before and after Diet Coke/Mentos reaction using a pH meter with a 2 point calibration. It was the same, 3.0. • Also, none of the ingredients in Mentos are basic: sugar, glucose syrup, hydrogenated coconut oil, gelatin, dextrin, natural flavor, corn starch, The classic baking soda and vinegar acid-base reaction produces unstable carbonic acid that rapidly gum arabic. decomposes into water and carbon dioxide, which • But you can make a fun escapes...
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